The hypersonic environment is extreme, involving both high temperature and oxidizing environment. In testing models in hypersonic ground facilities (or in flight), it is desirable to obtain real-time assessment of dynamic pressure and shear force (skin friction) at various places along the flow path. Furthermore, velocity profiling and flow uniformity of the hypersonic flow, at the exit plane of a hypersonic ground facility nozzle needs to be regularly assessed, preferably in real-time during testing, to better interpret external influences on test model performance.
The high speed, high temperature environment that exists in hypersonic flow applications also eliminates all non-intrusive velocity measurement techniques that rely on physical tracer particles to be propagated by the flow (such as particle image velocimetry or laser Doppler velocimetry). Multi point measurements of temperature and/or velocity using CARRS or Rayleigh molecular scattering suffer from the high cost of setup that accompanies these techniques and susceptibility to background noise/vibration, which limits their practicality towards implementation in ground test facilities.
In connection with pressure sensors, current technology in high temperature dynamic pressure sensors is limited to at most 600° C., with the need for active cooling in higher temperature situations. Active cooling is undesirable because it makes the sensors bulky and logistically challenging to integrate, while at the same time sacrificing performance.
Dynamic pressure measurements require sensing elements to be in direct contact with the fluid flow. Typically, this involves a surface with a hole aligned with the fluid flow boundary and a tube attached to the hole and leading to a pressure sensor with a diaphragm suspended in a cavity. In hot environments however, this tube is necessarily long in order to partially thermally isolate the pressure sensor. Such separation between the sensor and fluid flow creates a phase lag and signal attenuation—the tube and cavity act as a low pass filter—rendering dynamic pressure measurements problematic. While surface mounted pressure sensors do exist, they are not capable of operating at high temperatures because conversion of pressure into a signal relies on electrical circuitry that is either integrated into the sensing element, or in close proximity. Therefore, the sensor's operating envelope is restricted by the relatively low temperature capability of the electronics and electrical connections.
What is needed are optically coupled sensors that are capable of operating in ultra harsh environments, such as, the hypervelocity, high temperature, oxidizing environment of a high Mach number nozzle and the flow it produces, or the flow environment external (and internal within its flowpath) a test article flying at hypersonic speeds realizes.
A brief abstract of the technical disclosure in the specification is provided as well only for the purposes of complying with 37 C.F.R. 1.72. The abstract is not intended to be used for interpreting the scope of the claims.